With the increase in the development and use of mobile devices, the demand of secondary batteries as an energy source is growing. Among the secondary batteries, lithium batteries with high energy density and discharge voltage have been actively researched and have been commercially available. Also, recently, with the growing interest in environmental issues, many studies are conducted on electric vehicles (EVs) and hybrid electric vehicles (HEVs) other than vehicles running on fossil fuels, such as gasoline vehicles and diesel vehicles being attributable to air pollution. These electric vehicles and hybrid electric vehicles largely use Ni-metal hydride secondary batteries as a power source, and also lithium secondary batteries having high energy density and discharge voltage are actively researched for their use and have reached commercialization.
The lithium secondary batteries generally comprises an electrode assembly consisting of a cathode, an anode, and a separator interposed therebetween, and a non-aqueous electrolyte solution comprising a lithium salt and being impregnated in the electrode assembly, each of the cathode and the anode having an active material coated on a current collector. As the cathode active material, lithium-containing cobalt oxides, lithium-containing manganese oxides, lithium-containing nickel oxides, lithium composite oxides have been used. As the anode active material, carbon materials have been largely used.
In order for such a lithium secondary battery to be used as a power source of cellular phones, notebook PC and PDA, and further electric vehicles and hybrid electric vehicles, it should be operated even under severe conditions. For example, mobile devices and electric vehicles being used in the outside should be operated at a very low temperature during winter, and therefore the power source thereof is required to have good output characteristics at a low temperature. If the output is insufficient at a low temperature, a power system may be abnormally operated, and it is difficult to reach the minimum output necessary to starting, so making vehicle driving be impossible.
The output of the lithium secondary at a low temperature has been improved by largely modifying an electrolyte solution or an anode material. For example, in a lithium secondary battery used in HEV, amorphous carbon has been adopted as an anode active material so as to improve output at a low temperature. However, such a modification of an active material may change the applicable region of cell voltage or the appearance of voltage profile, thereby deteriorating high-temperature characteristics and reducing battery capacity.
Therefore, there is needed a technique capable of improving low-temperature output characteristics easily without deteriorating high-temperature characteristics and reducing battery capacity